The present invention relates to diagnostic medical devices which utilize ultrasound to determine the direction and speed of a fluid flowing in a vessel, and more particularly to continuous wave Doppler systems for measuring blood flow.
The simplest and most widely used Doppler systems to measure blood flow are xe2x80x9cCWxe2x80x9d (Continuous Wave) Doppler Probes. As shown in FIG. 1, a CW Doppler probe consists of two transducers 1 and 2, one continuously excited by a driving source (transducer 1), and the other continuously receiving (transducer 2).
The signals received by the second transducer can only originate where the first transducer""s xe2x80x9cbeamxe2x80x9d (the region in space where the first transducer produces acoustic energy) intersects the second transducers xe2x80x9cbeamxe2x80x9d (the region of space from which the transducer is sensitive, analogous to an antenna""s beam plot). These xe2x80x9csend beamsxe2x80x9d and xe2x80x9creceive beamsxe2x80x9d are identical for identical transducers by the principle of reciprocity, as is well-know in the state of the art.
In a CW Doppler system, the signal from transducer 2, the receiving transducer in FIG. 1, is filtered so that only signal energy at frequencies that have been Doppler-shifted away from the frequency transmitted by transducer 1 are further processed. One such method is known, where the received signal is heterodyned by the same frequency transmitted and a high-pass filter allows only difference frequencies to be passed. There are a number of such methods that are well-known, and may be found, for example, in xe2x80x9cDoppler Ultrasound and Its Use in Clinical Measurementxe2x80x9d, by P. Atkinson and J. P. Woodcock, Academic Press, London, 1982. (Electronic circuits for physical embodiments of CW Doppler systems are also shown there.) As is well known, the magnitude of the Doppler shift is related to the velocity of the moving material, e.g. blood, which causes Doppler shifted signals. Also, the power present at a particular frequency is proportional to the amount of transmitted power intercepted by the moving backscattering material and the efficiency of backscattering. For blood, for example, the amount of energy at different frequencies is related to the amount of blood moving at the corresponding velocity in the region where the transmitting and receiving beams coincide. (This region where the beams coincide is often described as the xe2x80x9csensitive regionxe2x80x9d in CW Doppler systems.)
That only blood moving in the sensitive region produces a signal in CW Doppler systems is both a strength and a liability. It is a strength in that there is no requirement for selecting a xe2x80x9csample gatexe2x80x9d as in pulsed wave Doppler systems, but a liability in that the sensitive volume is determined by the beam pattern of the transducers used and their geometrical arrangement, rather than by the setting of a sample gate. The simplicity in use of CW Doppler, as well as the simplicity of the system as a whole, has made CW Doppler a very widely used method of determining motion, in particular that of blood for medical purposes.
A significant drawback of Doppler blood measurement, however, is that the Doppler-shift frequency is proportional to the velocity of the blood multiplied by the cosine of the angle between the velocity vector and the beam. Therefore, Doppler measurements of blood velocity do not give absolute velocity values, but rather only relative values.
It is known that by using two or more beams at known angles to each other to insonate moving blood, two Doppler shift equations may be generated which contain the two unknowns of the velocity of the blood and the angle of the transducer to the velocity. Accordingly, since there are two (or more) equations with two unknowns, using algebra produced the velocity and the angle, independent of the angle of the transducer to the velocity vector.
A number of patents employ variations of this technique using multiple transducers at fixed angles to each other and using pulsed Doppler. Such patents include, U.S. Pat. No. 5,738,097, issued to Beach et al. U.S. Pat. No. 4,062,237 issued to Fox, used CW Doppler instead of pulsed Doppler, by employing multiple sets of transducers that operate at different frequencies from each other. Since each of the above patents disclose methods which demand multiple transducers that require significant electronic and mechanical alignment, these methods have not been found clinically useful.
It is also known that, by using a special transducer, namely a diffracting-grating transducer, multiple beams at known angles to each other may be produced. Commonly assigned U.S. Pat. Nos. 5,488,953 and 5,540,230 issued to the present inventor Vilkomerson, and incorporated herein by reference, disclose such a transducer and teach a method of determining the velocity and angle from these beams originating from the same transducer. Using a diffraction grating transducer structure and the disclosed method allows the velocity to be determined independently of the angle of the transducer to the blood velocity.
However, the system described in the previously mentioned patents still required pulsed Doppler operation, with the attendant need to set a sample gate to the region of interest. In the present invention there is disclosed a method of using diffracting-grating transducers in a CW Doppler system that does not require setting a sample gate, just like conventional CW Doppler systems, but provides angle-independent velocity determination with two transducers. Also disclosed are a number of other useful structures for angle-independent CW Doppler use.
As the previously mentioned patents teach how the information from the multiple beams is used to calculate the velocity of the moving blood (and if required the angle of the transducer to the blood vessel), what will be considered herein is the arrangement of transducers so as to obtain the multiple beam insonation needed to utilize those methods to calculate the blood velocity, but without the need to set a sample gate, i.e. using CW rather than pulsed Doppler.
A method of using diffracting grating transducers in a CW Doppler system for providing angle independent velocity determination. Such method is accomplished using only two transducers. An apparatus for determining angle-independent velocity comprising diffracting grating transducers operating in CW mode is also disclosed.
A method and apparatus for providing angle-independent CW probe operation obtained without diffraction-grating transducers is also disclosed.
An apparatus for determining the velocity of a fluid flowing through a lumen comprises a first diffraction grating transducer (DGT) responsive to a continuous wave (CW) input and operable in a first mode for producing a first signal beam at a first frequency, and in a second mode for producing a second signal beam at the first frequency; a second diffraction grating transducer (DGT) operating as a receiver and related to the first diffraction grating transducer at a predetermined angle (the xe2x80x9cdihedral anglexe2x80x9d), the second diffraction grating transducer producing a first receiving beam which intersects the first DGT first beam for receiving a first reflected signal associated with the first signal beam, and for producing a second receiving beam which intersects the first DGT second beam for receiving a second reflected signal associated with the second signal beam; the first beams adapted to intersect in a first predetermined region through which dynamic particles are undergoing velocity analysis, and the second beams adapted to intersect in a second predetermined region through which dynamic particles are undergoing velocity analysis; and electronic circuitry responsive to the first and second reflected signals for comparing with the first frequency to determine the velocity of the fluid.
A method for determining the velocity of a fluid flowing through a luen comprising directing a CW transmitting first diffraction grating transducer toward the lumen; directing a CW receiving second diffraction grating transducer toward the lumen, the first and second CW diffraction grating transducers oriented at a predetermined angle from one another such that corresponding edges of the DGTs converge to define an apex, producing first and second signal beams, respectively from each of the first and second DGTs which intersect at portions of the lumen, the first signal beam resulting from the first DGT transmitting at a first frequency and first phase, and the second signal beam resulting from the first DGT transmitting at the first frequency and second phase; the first and second signal beams resulting from the second DGT being Doppler shifted signals associated with the first DGT first signal beam and the first DGT second signal beam respectively, and determining the fluid velocity flowing through the lumen based on the Doppler shifted signals, the first frequency and the diffracting angle of the DGT.
There is also embodied in the present invention an apparatus for determining the velocity of blood flowing through a lumen comprising a first pair of continuous wave non-diffraction grating transducers coupled at a first dihedral angle relative to one another, the first pair of transducers producing beams which intersect at a given point which defines a sensitive volume; a second pair of continuous wave nondiffraction grating transducers coupled at the first dihedral angle relative to one another; the second pair of transducers producing beams which intersect at the given point, the first and second pairs of transducers oriented at a second angle B relative to one another; a signal source for driving the first and second pairs of transducers to cause one of the first pair to be driven at a first frequency and one of the second pair to be driven at the first frequency; whereby the other of the first pair receives a first reflected signal shifted in frequency relative to the signal from the first pair transmitting transducer, and whereby the other of the second pair receives a second reflected signal shifted in frequency relative to the signal from the second pair transmitting transducer; and electronic circuitry coupled to the first and second pairs of transducers for determining the velocity of the blood taking into account the frequency shifted reflected signals and driving signal frequencies, dihedral angles of the pairs of transducers, and angle B between the first and second pairs of transducers.
Also embodied in the present invention is a method of determining the velocity of a lumen comprising orthogonally orienting a first pair and a second pair of diffraction grating transducers relative to one another, each transducer of the first and second pairs positioned substantially adjacent one another and having edges converging toward one another so as to form dihedral angles, each the pair of transducers oriented so as to have intersecting beams which produce a sensitive volume around the lumen; driving a corresponding one of the first and second pairs of transducers to transmit at a first frequency such that a reflected signal indicative of the first frequency is received by the other of the first and second pairs; adjusting the driving frequency by a predetermined amount and driving the corresponding one of the first and second pairs of transducers to transmit the adjusted frequency such that a reflected signal indicative of the adjusted frequency is received by the other of the first and second pairs; determining the velocity in the two orthogonal directions based on the frequencies and dihedral angles for each of the first and second pairs of transducers; and calculating the velocity of the blood based on the velocity in the two orthogonal directions.